ln the prior art, it is known that objects which are to be exposed to reactive atmospheres at high temperatures may be rendered relatively inert, as compared to the base material, by deposition of a coating of metallic silicon or silicon oxide on the surface of the metallic article exposed to the reactive atmosphere and/or high temperature. In view of the fact that silicon dioxide has a high melting point, is unreactive toward many common atmosphere systems and has little catalytic activity, provision of such coatings is highly desirable. The fact that silicon dioxide has little catalytic activity has great value in such applications as equipment for steam cracking of hydrocarbons to produce ethylene. Secondary reactions which might result in the deposition of carbon on heat exchanger tubes are minimized with a silicon oxide coating on the exposed metallic surfaces in such reactors.
A number of processes are known and available for producing a siliconized surface on a metal, either to produce a silicon-rich or a silica coating. These methods are:
1. Molten metal or salt baths; PA1 2. Pack cementation which transfers silicon to the metal by generating a volatile silicon compound in-situ by reaction between pack solids and a gas; PA1 3. Slurry/sinter, by which a slurry of silicon-containing powder is applied to a metal, dried and sintered to produce a silicon coating. In this category, silica coatings are produced by deposition of silica solids such as sols or sol gel and sintering. PA1 4. Chemical vapor deposition of silicon via a gaseous or vaporized silicon compound; PA1 5. Chemical vapor deposition of silica via gaseous silicon and oxygen sources; PA1 6. Thermal spray of melted, atomized silicon-containing material on a metal substrate; PA1 7. Ion implantation of silicon; PA1 8. Physical vapor deposition of silicon or silicon oxide.
Chemical vapor deposition of silicon is one of the most desirable processes for a number of reasons, including such factors as uniform coating of the substrate, relatively low application temperatures and the option of forming a silicon diffusion layer, minimum cleaning of parts after treatment, no high-vacuum requirement and the fact that the parts are amenable to continuous processing, ease of surface cleaning and post treatment. In particular, silane (SiH.sub.4) is an attractive source of silicon because it is a gas containing only hydrogen and silicon thus avoiding problems caused by other gaseous or gasified silicon species such as the corrosion of process equipment or volatilization of the substrate by halide and other reactions that prevent formation of a diffusion coating such as carbon deposition and formation of silicon dioxide.
With processes involving the reaction at the surface of the object being coated, with a silicon halide such as SiCl.sub.4, Si.sub.2 Cl.sub.6, etc., and hydrogen, the overall reaction results in the formation of metallic silicon and hydrogen chloride. Silicon applied in this manner at temperatures greater than 1,000.degree. C. (1832.degree. F.) tends to diffuse into the substrate metal to form solid solutions and intermetallic compounds. These diffused coatings are especially desirable because there is no abrupt discontinuity in either composition or mechanical properties between the underlying substrate and the silicon at the surface. However, halogen-based processes suffer from a number of drawbacks centered around the reactivity and corrosivity of hydrogen chloride and other halogen derivatives. For example, iron chloride, which may be formed in the reaction, is volatile and loss of material and/or alteration of the composition of the substrate may be serious.
Another method of depositing metallic silicon is by the thermal decomposition of silane (SiH.sub.4) to yield silicon metal and hydrogen. British Patent No. 1,530,337 and British Patent Application No. 2,107,360A describe methods of applying protective coatings to metal, metal with an oxide coating, or to graphite. Critical surfaces in nuclear reactors are protected from oxidation by coating with silicon at greater than 477.degree. F. (250.degree. C.) under dry, nonoxidizing conditions followed by oxidizing the coating at a similar temperature, but under conditions such that silicon oxidizes faster than the substrate. For example, the patentees point out in the '337 patent that the 9% chromium steel was first dried in argon containing 2% hydrogen by heating to approximately 842.degree. F. (450.degree. C.) until the water vapor concentration in the effluent was less than 50 ppm followed by an addition of silane to the gas stream wherein the chromium steel in the form of tubes was treated for 24 hours at temperatures between 909.degree. and 980.degree. F. (480.degree. C. to 527.degree. C.). When treated for 6 days with a mixture containing 100 ppm of water vapor, the tubes exhibited a rate of weight gain per unit area less than 2% that of untreated tubes when exposed to carbon dioxide at 1035.degree. F. (556.degree. C.) for up to 4.000 hours. These are overlay coatings in contrast to the diffusion coatings prepared using silicon halide described above. For example, in patent application '360A, the applicants point out the importance of limiting the interdiffusion of Si with compounds of the substrate. These overlay coatings require long deposition times for their preparation. It is possible to form Si diffusion coatings using SiH.sub.4 but this requires higher temperatures. French workers produced diffusion coatings (solid solutions and metal silicides) utilizing silane under static conditions at elevated temperatures. [A. Abba, A. Galerie, and M. Caillet, Materials Chemistry, Vol 5, 147-164 (1980); H. Pons, A. Galerie, and M. Caillet, Materials Chemistry and Physics, Vol. 8, 153 (1983 ).] For iron and nickel, these temperatures were as high as 1100.degree. C. (2012.degree. F.). Others have produced metal silicides using silane on nickel using sputter-cleaned metal surfaces under high vacuum conditions. [L. H. Dubois and R. G. Nuzzo. J. Vac. Sci and Technol., A2(2), 441-445 (1984).]